1. Field of the Invention
Embodiments of the present invention generally relate to techniques for designing and manufacturing integrated circuits (ICs). More specifically, embodiments of the present invention relate to evaluating the quality of an assist feature placement in a mask layout based on a focus-sensitive cost-covariance field.
2. Related Art
The dramatic improvements in semiconductor integration densities in recent years have largely been made possible by corresponding improvements in semiconductor manufacturing technologies.
One such manufacturing technology involves placing assist features in a mask layout to improve the depth of focus of the patterns on the mask layout which are intended to be printed on the wafer. Such assist features can be printing assist features (e.g., super-resolution assist features) or non-printing assist features (e.g., sub-resolution assist features).
Existing techniques for placing assist features typically use mask rules, which place and clean up assist features based on combinations of feature width and spacing parameters. Unfortunately, such rule-based approaches can result in missed or sub-optimal placement and/or clean up of assist features. Furthermore, the complexity of such rules increases rapidly with shrinking feature size, thereby requiring increasing effort on the part of designers. Moreover, these rules can be overly restrictive, which can prevent designers from achieving optimal semiconductor device performance.
In contrast to the rule-based approaches, model-based assist feature (AF) placement techniques have the advantages of facilitating faster adaptation to new manufacturing processes, achieving “smoother” AF placement, and reducing the need for aggressive and complicated clean-up and placement rules.
For example, one model-based AF placement technique has been successfully used to generate assist features for contact hole layers based on an optical model. However, because this placement technique for the contact hole layers is based on maximizing intensity at the center of contacts, the technique often produces incorrect AF placement solutions when it is applied to line-space layers.
Another model-based AF placement technique uses the inverse lithography technique (ILT) to produce assist features. Such assist features are typically generated by constructing cost functions that maximize image log slope (ILS) at pattern edges. Unfortunately, ILT-based AF placement techniques not only require many iterations to converge on a solution but often produce poor AF placement results.
Hence, what is needed is a method and apparatus for determining an AF placement to improve the depth of focus for a layout without the above-described problems.